Chem 332L Lab Quiz Review
Laboratory Practices and Safety Protocols
Maintaining a clean and safe laboratory environment is fundamental to organic chemistry. Good practices include keeping the lab area clean and dry at all times and ensuring hands are washed thoroughly after each experiment is complete. Regarding chemical management, unused chemicals should never be returned to the original reagent bottles; if an excess of a reagent is taken, it must be disposed of as waste to prevent contamination of the stock supply. Specific safety hazards, such as those associated with diethyl ether, must be addressed. Diethyl ether is highly volatile and evaporates easily at room temperature; because it can easily ignite at higher temperatures, it is critical to avoid open flames when working with this substance.
Analytical Techniques: Nuclear Magnetic Resonance (NMR) Spectroscopy
Nuclear Magnetic Resonance (NMR) spectroscopy is used to determine the structure of organic molecules by analyzing the environment of hydrogen atoms. Key metrics include chemical shift, integration, and splitting patterns. In the aromatic region, hydrogens typically appear at chemical shifts between $7\,ppm$ and $8\,ppm$. Integration values correspond to the number of hydrogens responsible for a specific peak. The splitting pattern, or peak shape (e.g., singlet, doublet, triplet, quartet), is determined by the $n+1$ rule, where $n$ is the number of neighboring hydrogens on adjacent carbons. For instance, in $2$-methyl-$2$-hexanol, the methyl groups at position B have zero neighboring hydrogens, resulting in an integration of $6H$ (for two identical methyl groups) and a splitting pattern of a singlet ($0+1=1$). Conversely, at position F, the methyl group has two neighboring hydrogens, leading to an integration of $3H$ and a triplet splitting pattern ($2+1=3$).
Index of Hydrogen Deficiency and Structural Isomerism
The Index of Hydrogen Deficiency (IHD), or degree of unsaturation, is calculated using the formula . For a substance with the chemical formula $C_5H_{10}O$, the calculation is . An IHD of $1$ signifies the presence of one double bond (such as a $C=C$ or $C=O$ carbonyl group) or one ring. For disubstituted aromatic rings, three constitutional isomers are possible based on the relative positions of the substituents: Ortho ($1,2$-substitution), Meta ($1,3$-substitution), and Para ($1,4$-substitution). In nomenclature, these positions define the spatial relationship, such as a nitro group being meta to another substituent on a benzene ring.
Thin Layer Chromatography (TLC) and Reaction Monitoring
Thin Layer Chromatography (TLC) is a technique used to separate components of a mixture and monitor reaction progress based on polarity. It utilizes a stationary phase (a polar plate) and a mobile phase (a solvent). Compounds are governed by the principle of "like dissolves like"; nonpolar compounds dissolve better in nonpolar solvents and rise higher on the plate, while polar compounds adhere more strongly to the polar stationary phase and remain closer to the bottom. Consequently, the most polar compound is the one with the lowest $R_f$ value.
In practice, many compounds are colorless and must be visualized using methods such as an Iodine chamber, which can turn spots orange, or UV light. When monitoring a reaction, a TLC plate is spotted with samples such as the crude product (Cd), filtrate (Fl), purified crystals (Cs or Pd), or mixtures (M). If a spot corresponding to the starting material remains in the reaction lane, the reaction has not gone to completion. The disappearance of the starting material spot indicates the reaction is finished.
Grignard Reactions and Limiting Reagent Calculations
The Grignard reaction involves the formation of a carbon-carbon bond using a Grignard reagent (an organomagnesium halide). In the general mechanism, the Grignard reagent acts as the nucleophile (electron-rich species, electron donor, $\delta^-$), while the aldehyde or ketone acts as the electrophile (electron-deficient or electron-poor species, electron acceptor, $\delta^+$). A common step that must be avoided during the preparation is the addition of water to the Grignard reagent, as water will protonate the reagent ($R-MgBr + H_2O \rightarrow R-H + MgBrOH$), destroying the nucleophile.
To determine the limiting reagent in a synthesis, such as the preparation of $2$-methyl-$2$-hexanol, one must calculate the moles of product possible from each reactant. For a reaction using $2.95\,g$ of Magnesium (MW: $24.305\,g/mol$), $3.5\,mL$ of butyl bromide (MW: $137.02\,g/mol$, density: $1.27\,g/mL$), and $3\,mL$ of acetone (MW: $58.08\,g/mol$, density: $0.784\,g/mL$):
In this scenario, butyl bromide is the limiting reagent because it yields the lowest amount of product ($0.0324\,mol$), which represents the theoretical yield.
Oxidation, Reduction, and Polarimetry
Oxidation in organic chemistry is defined as a reaction where a carbon atom gains bonds to electronegative atoms, particularly oxygen, or loses bonds to hydrogen. Conversely, reduction involves increasing the number of $C-H$ bonds or decreasing the number of $C-O$ bonds ($C-H$ bonds increase, $O-H$ bonds decrease). For example, reacting a ketone like cyclopentanone with sodium borohydride ($NaBH_4$) is a reduction that yields a secondary alcohol. In the synthesis of $9$-fluorenone from $9$-fluorenol, the process is an oxidation because the product has more bonds to oxygen.
Polarimetry is used to measure the optical rotation (chirality) of a substance. The specific rotation is a standardized value calculated using the equation:
where $\alpha$ is the experimentally measured observed rotation, $c$ is the concentration in $g/mL$, and $l$ is the path length in decimeters ($dm$). For a sample where $0.1\,g$ is dissolved in $10\,mL$ of chloroform ($c = 0.01\,g/mL$) and measured in a $1.00\,dm$ tube with an observed rotation of $+1.05^\circ$, the specific rotation is:
Fischer Esterification and Saponification
Fischer esterification involves the reaction of a carboxylic acid with an alcohol in the presence of an acid catalyst to form an ester and water. This is an equilibrium reaction with a constant close to $1$. To shift the equilibrium toward the products (right) and increase yield, one can use an excess of one reagent (alcohol or acid) or remove water as it forms. The reverse process, acid-catalyzed hydrolysis of an ester, yields the original alcohol and carboxylic acid.
Saponification is the base-promoted hydrolysis of an ester, often using a strong base like sodium hydroxide ($NaOH$). The first step of the mechanism is nucleophilic addition to the carbonyl. This process produces an alcohol and a carboxylate salt. The carboxylate molecule acts as a surfactant or soap, possessing a hydrophilic head (the charged carboxylate group containing oxygen) and a hydrophobic tail (the long, uncharged hydrocarbon chain).
Carbonyl Condensations and Rearrangements
An Aldol condensation involves the reaction of a ketone and an aldehyde. One reagent must form a reactive enolate species, which then undergoes nucleophilic addition. The initial aldol addition product undergoes dehydration (loss of water) to form an $\alpha,\beta$-unsaturated carbonyl (an alkene group). IR spectroscopy is used to characterize these products, confirming the presence of conjugated carbonyls and alkene signals. Melting point analysis can assess purity; for example, if $(E,E)$-dibenzylideneacetone is expected to melt at $104-107^\circ C$, a measured value of $85^\circ C$ indicates impurities, a wet product, an incomplete reaction, or the formation of a different isomer.
Benzilic acid rearrangement occurs when benzil is converted to benzilic acid via a $1,2$-phenyl migration that changes the carbon skeleton. This reaction is typically performed under reflux to maintain a constant temperature without losing solvent. $^1H$ NMR is often insufficient for monitoring this specific reaction because both reactant and product contain only aromatic protons, making their spectra difficult to differentiate. IR is preferred as it clearly shows the formation of the carboxylic acid group. During workup, the mixture is acidified to a $pH$ of $2$ using $HCl$ to ensure the full protonation and precipitation of the benzilic acid from its salt form.
Radical Polymerization and Catalysis
Polymerization can be achieved through radical processes involving three main steps: initiation, propagation, and termination. Controlled Radical Polymerization, such as Organocatalyzed Atom Transfer Radical Polymerization (O-ATRP), is used to create polymers with uniform chain lengths and narrower distributions than free radical polymerization. This control is achieved by keeping radical concentrations low, which prevents uncontrolled termination (such as radical-radical coupling or disproportionation) and allows for uniform chain growth.
In photoredox polymerization, light (e.g., sunlight) serves as the energy source, where a catalyst like perylene absorbs light to enter an excited state. In a typical reaction mixture, different components serve specific roles: methyl methacrylate (MMA) acts as the monomer, ethyl-$\alpha$-bromophenyl acetate (EBPA) as the initiator, perylene as the catalyst, and ethyl acetate as the solvent. Isolation of the polymer product, such as poly(methyl methacrylate) (PMMA), is often achieved through precipitation; since PMMA is insoluble in methanol, adding methanol to the reaction mixture causes the polymer to crash out as a solid. Catalysts in these reactions speed up the process by lowering activation energy and are regenerated at the end of the cycle.
Questions & Discussion
Q: Which of the statements below is NOT considered good practice in a chemistry lab?
A: Return unused chemicals to the reagent bottle. (This is bad practice).
Q: What is the purpose of conducting a reaction under reflux?
A: Reflux allows a reaction to be heated at its boiling point for an extended period of time without losing solvent to evaporation, because vapors condense in the condenser and drip back into the reaction flask.
Q: Describe how at least one byproduct or solvent was removed from the reaction mix to purify 9-fluorenone.
A: Sodium carbonate removed acidic impurities; water washes removed salts ($Na^+$, $Cl^-$); magnesium sulfate acted as a drying agent; and the rotovap removed hexanes.
Q: Which intermolecular force is present in glucose but absent from α-glucose pentaacetate?
A: Hydrogen bonding. Glucose has many $-OH$ groups, while the product replaces these with acetate groups.
Q: Identify the steps in a radical mechanism.
A: 1. Deactivation (radical neutralized/capped); 2. Activation (radical created); 3. Propagation (monomers added to the chain).